That is a really tremendously beautiful 71 Mach 1. I am very happy you were able to getit into an enjoyable state, they really are nice cars when set up correctly. I get not wanting to change the 69 T-Bird, as we have kept our 69 Shelby GT500 in pure stock form. It is plenty fast as it is, there is no need or desire to goose more power out of it. We also have a 2020 GT500 we are keeping in stock form, largely because I do not want to do anything that may compromise its extended warranty, and with 760 HP coming from it I see no need for even more power being put on tap.
We have a 73 Mustang Convertible with a 3-02 2v, C4 auto tranny, and a very steep 2.79:1 rear axle gears. It is being kept stock as it has only 21,000 original miles on it (barn stored over 40 years), and with those steep rear axle gears we would need to work with a lower set of gears (TractionLok) if we were to want to alter the engine, which is running perfectly well as it is.
With our 73 Mach 1, things are a bit different. The prior owner replaced its base 302 2v with a 1994 F-150 351W (not C), which was built to a moderate street/strip level of performance. He also replaced the rear ale gears with a 3.5:1 TractionLok unit. We swapped an AOL in to replace the C4. There not much else I want to do with the 73 Mach 1. We already installed a Classic Air A/C system (on both 73 Mustangs), and had the alternator rebuilt with a rotor with more windings in it to go from 42 amps to 65+ amps just to handle all the electrical load from the goodies we added (including a dual electric fan cooling system on both 73s). When we got the Mach 1 it was a nice riding vehicle, although the 3.5:1 rear axle ratio spun the engine at 2,700 or so RPM at 60 MPH, a little more RPM than I wanted. The C4 began to slip in its WOT 1-2 upshift. I replaced it with an AOD to retain the 1st, 2nd, 3rd gear performance, plus get the 0.7:1 OverDrive 4th gear to both lower the highway cruising engine RPM, and to reclaim the top end the lower rear axle gear ratio took away. Since the engine has plenty of power at the top RPM ranges it can push through the air resistance found at 100+ MPH speeds, so the OD gear was not wasted. And, at 60 MPH in 4th OD gear the engine RPM is running at a very nice 1,900 RPM or so. now cruising at highway speeds is much more enjoyable. And like your 71 Mach 1, after a deep re$toration it is one terrific looking car. Yet the two Shelby GT500s are my two most favored cars in our little fleet, where the 60 Shelby is my all time favorite car ever.
Our 73 Mach 1: